The present application claims priority to Application No. 102 01 016.1, filed in the Federal Republic of Germany on Jan. 11, 2002, which is expressly incorporated herein in its entirety by reference thereto.
The present invention relates to a method for operating an internal combustion engine with exhaust gas purification system, in which a rich mode of the internal combustion engine with a rich exhaust gas composition and a lean mode of the internal combustion engine with a lean exhaust gas composition are set alternately, and by the exhaust gas purification system with a rich exhaust gas composition ammonia is synthesized and stored and with a lean exhaust gas composition nitrogen oxides are reduced to form nitrogen by the stored ammonia.
The present invention also relates to an internal combustion engine with exhaust gas purification system for performing the method according to the invention.
Nitrogen oxides in oxygen-rich combustion exhaust gases can be reduced very successfully to form nitrogen with the aid of ammonia. In this context, it is appropriate for the ammonia to be generated in a catalytic converter unit in the vehicle itself. The ammonia-generating catalytic converter unit may be formed, for example, by a three-way catalytic converter which, with a substoichiometric exhaust gas composition, synthesizes ammonia NH3 from nitrogen oxides NOx and hydrogen H2. The ammonia is fed into a downstream nitrogen oxide reduction catalytic converter when the exhaust gas composition is substoichiometric or rich. When the exhaust gas composition is subsequently superstoichiometric, or lean, this ammonia reduces nitrogen oxides to form nitrogen in the nitrogen oxide reduction catalytic converter. The quantity of ammonia generated is dependent on the quantity of nitrogen oxides available during substoichiometric or rich combustion.
The term lean mode denotes a superstoichiometric engine mode, in which excess oxygen, i.e., xcex greater than 1, prevails during the combustion. The term rich mode is understood as meaning a substoichiometric engine mode, in which excess fuel, i.e., xcex less than 1, prevails during the combustion. Accordingly, a lean exhaust gas composition indicates an excess of oxygen in the exhaust gas, and a rich exhaust gas composition indicates an excess of fuel in the exhaust gas.
German Published Patent Application No. 198 20 828 describes an exhaust gas purification system which with a rich exhaust gas composition uses a first catalytic converter unit to generate ammonia from nitrogen oxides and hydrogen present in the exhaust gas and stores the ammonia which is generated in a second catalytic converter unit. With a lean exhaust gas composition, nitrogen oxides which are present in the exhaust gas are subjected to a reduction reaction using the temporarily stored ammonia as reducing agent. To ensure that sufficient nitrogen oxides are present in the exhaust gas even with a rich exhaust gas composition, in order to synthesize a significant quantity of ammonia, a third catalytic converter unit, which with a lean exhaust gas composition temporarily stores nitrogen oxides which are present in the exhaust gas and with a rich exhaust gas composition releases the previously stored nitrogen oxides again, is connected upstream of the first catalytic converter unit, which is intended to generate ammonia with a rich exhaust gas composition.
German Published Patent Application No. 197 50 226 describes an engine control unit for a diesel engine with exhaust gas purification system, which is provided with an adsorption device for nitrogen oxides NOx, which adsorbs nitrogen oxides with a lean exhaust gas composition. To regenerate the adsorber system, a rich exhaust gas composition, in which the exhaust gases have a reducing atmosphere, is required from time to time. To set a rich or lean exhaust gas composition, a quantity of fuel is injected by a preinjection, a main injection and an afterinjection of fuel. The afterinjection of fuel is substantially only evaporated and treated but is only burnt to a small extent, and consequently there is a greatly increased emission of unburned hydrocarbons HC and carbon monoxide CO. When the adsorption device releases the stored nitrogen oxides when there is a rich exhaust gas composition, the reaction products hydrocarbon HC and carbon monoxide CO on the adsorber surfaces are used to convert nitrogen oxides into nitrogen N2.
It is an object of the present invention to provide a method for operating an internal combustion engine and also an internal combustion engine in which, by influencing the combustion of fuel in the internal combustion engine, a quantity of nitrogen oxides which is sufficient for the synthesis of significant quantities of ammonia is provided even with a rich exhaust gas composition.
The above and other beneficial objects of the present invention are achieved by providing a method and an internal combustion engine as described herein.
The present invention provides a method for operating an internal combustion engine with exhaust gas purification system, in which a rich mode of the internal combustion engine with a rich exhaust gas composition and a lean mode of the internal combustion engine with a lean exhaust gas composition are set alternately, and by the exhaust gas purification system with a rich exhaust gas composition ammonia is synthesized and stored and with a lean exhaust gas composition nitrogen oxides are reduced to form nitrogen by the stored ammonia, in which method the quantity of fuel which is introduced during a combustion cycle of the internal combustion engine is introduced by at least one preinjection, at least one main injection and at least one afterinjection, the at least one preinjection and the at least one main injection are burnt under excess oxygen in order to generate a quantity of nitrogen oxides which is sufficient for the ammonia synthesis, and a rich or lean exhaust gas composition is set by the at least one afterinjection.
By these measures, it is possible to generate a sufficient quantity of nitrogen oxide even in the event of rich combustion throughout the entire combustion operation and a rich exhaust gas composition resulting therefrom, so that a significant quantity of ammonia may be synthesized during rich mode. Since significantly greater quantities of nitrogen oxides are available compared to conventional methods for producing a rich mode for the ammonia synthesis, the duration of the rich-mode phases may be shortened. Furthermore, there may be no need for measures for temporarily storing nitrogen oxides during the lean mode and subsequently releasing them in rich mode.
It may be provided that the at least one afterinjection is at least partially burnt in the combustion chamber and contributes to the engine load.
The total quantity of fuel injected may contribute to the engine load. It is also possible to reach very high exhaust gas temperatures more quickly than with standard diesel engine combustion. Such high exhaust gas temperatures may be required in an ammonia-generating catalytic converter in order to synthesize ammonia from nitrogen and hydrogen.
The combustion of the quantity of fuel introduced during a cycle of the internal combustion engine may occur in rich mode without exhaust gas recirculation.
As a result, the preinjection quantities and the main injection quantity may be converted in the combustion chamber with a very high oxygen excess, with the result that a significantly higher formation of nitrogen oxide in the combustion chamber may occur compared to standard combustion with exhaust gas recirculation. Nevertheless, it is possible to set a rich combustion with a rich exhaust gas composition by the at least one afterinjection.
At least one first afterinjection, which burns in the combustion chamber, and at least one further, offset afterinjection may be provided in order to set a lean or rich exhaust gas composition.
A further, offset afterinjection may allow particularly flexible and at the same time accurate setting of the exhaust gas composition. An accurate setting of the engine load may be effected by the first afterinjection.
The starts of injection of the preinjections in rich mode may substantially correspond to the starts of injection in lean mode. The starts of injection of the main injections in rich mode may substantially correspond to the starts of injection in lean mode.
These measures may allow the transition between lean mode and rich mode to occur without being noticed by a driver of a motor vehicle having the internal combustion engine. Furthermore, this selection of the starts of injection may also promote generation of nitrogen oxides in rich mode.
The problem on which the present invention is based may also be solved by providing an internal combustion engine with exhaust gas purification system for performing the method according to the present invention, in which the exhaust gas purification system is provided with a first catalytic converter unit, which with a rich exhaust gas composition generates ammonia from corresponding exhaust gas constituents, and with a second catalytic converter unit, which is connected downstream of the first catalytic converter unit and with a rich exhaust gas composition temporarily stores ammonia which is generated by the first catalytic converter unit and with a lean exhaust gas composition subjects nitrogen oxides which are present in the exhaust gas to a reduction reaction using the temporarily stored ammonia as reducing agent, in which internal combustion engine an injection system having an arrangement configured to introduce a quantity of fuel by at least one preinjection, at least one main injection and at least one afterinjection, and an arrangement configured to set an excess of oxygen in rich mode during the combustion of the at least one preinjection and at least one main injection in the combustion chamber, and an arrangement configured to alter the time, quantity and duration of the at least one afterinjection in order to set a rich or lean exhaust gas composition.
These measures may make it possible, in the exhaust gas purification system, to dispense with a nitrogen oxide adsorber, which stores nitrogen oxides during a lean mode and releases them again during rich mode, or to reduce the size of a nitrogen oxide adsorber of this type. Since nitrogen oxide adsorbers of this type may also store oxygen, which in the phases with a rich mode may delay the release of the stored nitrogen oxides, dispensing with or reducing the size of a nitrogen oxide adsorber of this type may make it possible to shorten the rich-mode phases and thereby to reduce the increased fuel consumption resulting from these rich-mode phases. To set an excess of oxygen during the combustion of the preinjections and main injections, it is possible, for example, to reduce an injections quantity, open a throttle valve in the intake path and/or close an exhaust gas recirculation.
Further features and aspects of the present invention are described in the following description of an example embodiment of the present invention with reference to the drawing.